3.1 Fiber analysis of ma textile sample
LM was used to identify the fiber type of ma textile sample. As shown in Fig. 3a and 3b, the morphology of standard ramie fiber is relatively thick (the average width of the fiber is 30-60 μm), one part of the fiber is shaped like a circular tube, and another part of the fiber is shaped like a flat long ribbon. It shows clear longitudinal stripes, obvious horizontal knots on the fiber wall, and a wine-red color [33, 34]. Careful morphology comparison between standard ramie fiber and fiber of painted ma textile sample, showed that the painted ma textile sample displays the same fiber morphology and color with standard ramie fiber. Therefore, it is deduced that the fiber type of ma textile sample is ramie fiber.
Fig.3 Fiber analysis of the painted ma textile: fiber morphology of ramie (a, LM×200; b, LM×400), fiber morphology of painted ma textile (c, LM×200; d, LM×400).
3.2 Preparation of reinforcement material
According to the fiber identification result and the principle of compatibility, the reinforcement material was prepared with a ramie net and PVA-217 adhesive. As shown in Fig. 4(a), the ramie thread with a diameter of 0.8~1 mm was woven as a net with a mesh of 3 mm×3 mm. Fig. 4(b) shows that the 5% concentration of the PVA-217 aqueous solution was prepared as the adhesive, and a spray gun was used to evenly spray the adhesive on the ramie net. The distance between the gun head and the ramie net is 3 cm, the spray pressure of the spray gun is 0.5 MPa~0.8 MPa, and the coating thickness of the adhesive is 15~20 μm. Finally, the sprayed ramie net was dried naturally, and cut out of the weaving frame.
Fig.4 Preparation of reinforcement net: spin a web on a webbing machine net (a), spray water-soluble adhesive on the woven net (b).
3.3 Durability evaluations of reinforcement material before and after aging
Conservation professional groups always employ reinforcement materials with good durability to carry out conservation methods. As the addition to the support fabric will be a permanent intervention to stabilize weakened areas and safely transport the painted ma textile, the durability of the reinforcement material should be evaluated before using it. In this work, the reinforcement materials were evaluated with three kinds of general accelerated aging: dry heat, wet heat, and ultraviolet light. As shown in Table 4, the retention rate of tensile strength of the ramie net after dry heat aging for 5 days and wet heat aging for 5 days is above 80%, and the retention rate of tensile strength after UV light aging for 5 days is above 90%. These aging results show that the ramie net as the reinforcement material possesses good durability for the support and protection of the painted ramie textile. Generally, the durability of the reinforcement material should not be higher than that of the original object, as that would put a strain and cause mechanical stress to the fragile object. Our reinforced ramie net possessed good compatibility with the original object to balance the strain and mechanical stress.
Fig. 5 showed that the wet heat aging and ultraviolet light aging had less effect on the color of the reinforcement materials, while the dry heat aging makes more obvious color changes of the PVA film and ramie net. Therefore, it is speculated that the discoloration of the ramie net was caused by the color change of the PVA after aging. The discoloration of the PVA film was caused by the thermal oxidation reaction of the PVA in the dry heat aging box at 105 °C, and some of the hydroxyl groups are oxidized to carbonyl groups, and the reaction mechanism is shown in Fig. 6.
Fig. 5 The color alteration data and effect pictures of several ramie nets with PVA-217 adhesive before and after aging several days by using three aging methods. The chromatic aberration data (white numbers) were listed in the effect pictures.
With the increasing aging time, the carbonyl content becomes high, and the color of the PVA film gradually changes from colorless to light yellow, and finally turns to yellow. Meanwhile, the PVA film weakens and becomes brittle and hard. Therefore, it is necessary to further improve the color alterations before and after dry heat aging of the reinforcement material, before it can be applied to cultural relics.
Fig.6 Reaction mechanism of PVA film during heating aging process.
3.4 Reversibility process
Retreat-ability is the main concern of conservators nowadays [35]. The water-soluble adhesive has good reversibility as a bonding agent for ramie net and painted ramie textile, and they are very stable bonding (Fig. 7a). The ramie net could be removed from painted ramie textile (Fig. 7b) after spraying an appropriate amount of ultra-pure water for a few minutes (Fig. 7c). During the above process, the painted ramie textile sample almost has no damage unless adhesive residues, and it had good reprocessing properties (Fig. 7d).
Fig. 7 Reversibility process: before (a) and after (d) removing reinforcement material, spray ultra-pure water (b), removal of reinforcement net (c).
3.5 Conservation process and result
Through the above scientific evaluation of the reinforcement material, the principles of compatibility, safety, durability, and reversibility for conservation have been considered. Hence, this reinforcement material was applied to the conservation of the painted textile. As shown in Fig. 14(a), this original painted ramie textile was decayed and broken before it was reinforced and restored. In order to preserve the original appearance of the cultural relics, some broken holes, breakages, and breaks were woven and conserved. For safe exhibition and stable preservation of this painted ramie textile, the back of this artwork was strengthened by ramie net, and ramie net was used as the supporting material, stabilizing weakened areas of the painted ramie textile for safe transport. On the other hand, supporting material was also used as a skeleton to stitch the broken parts and helped to fill the missing areas. Overall, the conservation treatment was successful (Fig. 14b). Reinforcement technology of the painted ramie textile in this paper would provide an important reference for the conservation of decayed and extensively damaged textiles. As shown in Fig. 8-13, the conservation process showed down as the following:
(1) Realignment of the distorted weave and mechanical cleaning: The painted ramie textile was laid on a table and use tweezers to arrange the damaged fibers on the edge one by one (Fig. 8a). The mechanical cleaning was done by fine and soft wool brush to remove loose dust and dirt (Fig. 8b).
Fig. 8 Using tweezers (a) and soft wool brush (b) to arrange the damaged fibers on the edge one by one.
(2) Consolidation of pigment in the colored painting: Because the objects would inevitably get into contact with water during the conservation process, in order to prevent loss of pigment particles, we performed the consolidation with 8% concentration of aqueous fluorine ethanol and water solution (Fig. 9a). Absorbent cotton swabs were dipped into an appropriate amount of water-based fluororesin solution, and then it was evenly applied on the colored painting (Fig. 9b), and left to air dry at room temperature.
Fig. 9 Consolidation process of pigment in the painted ramie textile.
(3) Wet cleaning and stain removal: We added ultra-pure water into the ultra-fine atomization sprayer, and sprayed slowly on the ramie fabric. With the spray, the ultra-pure water would be flowed out of the backing paper until the liquid flowing out was relatively clean (Fig. 10). Then we rolled it out with a rolled towel to make the objects flat on the tabletop.
Fig. 10 Wet cleaning and stain removal
(4) Fine realignment on the back of the painted ramie textile: The separated and warped threads were arranged in their original place with fine tweezers (Fig. 11). Because some of the weft threads were missing, the scattered weft threads were arranged in the original place before the reinforcement procedure.
Fig. 11 Fine realignment of the warped threads was based on the original texture of the fabric.
(5) Reinforcement of the ramie net: We coated the prepared ramie net with PVA-217 adhesive, and attached it to the back of the object and flattened it with a rolled wet towel (Fig. 12a). During this process, PVA-217 adhesive attached to the ramie net would be partly dissolved in water, thereby being sticky enough to adhere and support the object (Fig. 12b), and then left to air dry at room temperature.
Fig. 12 Reinforcement process of the ramie net: We attached the prepared ramie net to the back of the object (a) and flattened it with a rolled wet towel (b).
(6) In-fill of areas of loss: We used the ramie net as the support where the torn parts were stitched, and the missing areas were woven with aged ramie threads (Fig. 13). Aged ramie was preferred as its mechanical properties would be similar to those of the original object.
Fig. 13 The weave process of decayed ramie fabrics: before weaving (a), weaving with ramie threads (b), and after weaving (c).
Fig. 14 Effect pictures of decayed ramie fabrics before (a) and after (b) conservation.
3.6 Exhibition and collection
After conservation, the painted ramie textile has been stably exhibited for a long time, which is an important mean to evaluate the conservation materials and treatment. Period of time, the painted ramie textile has been monitored for safety and stability since 2016. The results showed no obvious discoloration, and it has been exhibited in the museum under stable temperature and humidity of 23 °C and 50% (Fig. 15), respectively. For the storage, we recommend that it should be unfolded and laid flat if the storage space permits.
Fig.15 Exhibition photo of the painted ramie textile in one museum.